1. Field of the Invention
The present invention relates to a lateral photo-sensing device used for photo-electric conversion or photo-detection, an opt-electronic integrated circuit using the lateral photo-sensing device and a photo-logic device using the lateral photo-sensing device. Further the present invention more particularly relates to a photo-conductive type lateral photo-sensing device used for an opt-electronic integrated circuit (OEIC) or a photo-module for light communication or light information processing, to a opt-electronic integrated circuit (OEIC) used in an optical local area network (LAN) a fiber distributed data interface (FDDI) which has a limited operation band and operates at a high speed, to a photo-receiving OEIC applicable to both long distance and short distance light communications, and to an optical switch for light communication which is usable in a photo-logic operation in an opt-neuro system or an optical computer.
2. Related Background Art
FIGS. 1A, 1B and 1C show examples of prior art lateral photo-sensing devices.
In a first photo-conductive type photo-sensing device shown in FIG. 1A (Japanese Patent Application No. 63-88758), a recess is formed in a semi-insulative semiconductor substrate 5, and a p-type semiconductor layer 6 is formed therein and then an n-type photo-sensing layer 7 is formed. In this manner, a flat lateral photo-sensing device which is easy to isolate is provided. A pair of n-type ohmic electrodes 9a and 9b are formed on the photo-sensing layer 7 as photo-sensing device electrodes. A p-type ohmic electrode 8 formed and the semiconductor layer 6 functions to apply a reverse bias between the semiconductor layer 6 and the photo-sensing layer 7. Holes generated in the photo-sensing layer 7 are collected by the ohmic electrode 8 to improve an operation speed of the device. Further, the photo-sensing layer is depleted to reduce a dark current.
In a second photo-conductive type photo-sensing device shown in FIG. 1B (Japanese Patent Application No. 63-81030), an n-type photo-sensing layer 11 is first grown on a semi-insulative semiconductor substrate 12.
Then, a pair of n-type ohmic electrodes 13a and 13b and a pair of p-type ohmic electrodes 14a and 14b surrounding the electrodes 13a and 13b are formed on the photo-sensing layer 11. The electrodes 14a and 14b function to increase a resistance of a light absorbing layer beneath the electrodes 13a and 13b so that a dark current is reduced and a performance of the device is improved.
In a third photo-conductive type photo-sensing device shown in FIG. 1C (pp 1. Phys. Lett. 54(1), Jan. 2, 1989, pp 16-17), a light absorbing GaInAs layer 24 is formed on semi-insulative InP substrate 21 and AlInAs layer 22 with an intervention of a superlattice layer 23. Then, a barrier enhancing AlInAs layer 26 and a Schottky electrode Al layer 27 are formed on the GaInAs layer 24 with an intervention of a superlattice layer 25. It is then mesa-etched to form a device as shown. Because of the presence of the AlInAs layer 26, the device has a small dark current and exhibits a high response characteristic.
In the first and second photo-conductive type photo-sensing devices described above, the n-type ohmic electrodes are used as the device electrodes. As a result, the device has a relatively large dark current and a large noise and it is not suitable for use as the OEIC. Further, it is not suitable for a high speed operation because of reinfection of carriers.
On the other hand, in the third photo-conductive type photo-sensing device, a responsivity and a response speed are low at a low bias voltage (e.g.-3v). In order to attain sufficient sensitivity and response speed in such a device, a high bias voltage such as vi or higher is required.
Further FIG. 2 shows a conventional circuit configuration used in a prior art photo-sensing OEIC or photo-sensing module. A pin-photo-diode (PD), a metal-semiconductor-metal (MSM) or an avalanche photo-diode (APD) is used as a photo-sensing device 32 for converting a signal light to an electrical detection signal. In order to amplify the detection signal of the photo-sensing device 32, an amplifier circuit comprising an amplifier 34 and a load resistor 36 is connected succeeding to the photo-sensing device 32. While a trans-impedance type amplifier circuit may be connected the electrical signal may be mixed by other methods.
However, when the pin-PD or the MSM is used in the OEIC, a signal detection band is governed by a capacitance and a time constant of a load resistor, and a photo-sensing diameter must be small in order to attain a high operation speed. In such an OEIC, a responsivity does not exceed 1 A/W in principle and no substantial amplification effect is attained. Thus, the amplifier circuit is necessarily complex. In the OEIC which uses the APD, the drive voltage is several tens of volts, which makes it difficult to apply to the OEIC which is operated by a signal power supply. In the approach to mix the electrical signal, the circuit configuration is difficult to design and high integration density OEIC is difficult to attain.
Further more, fibers used for light communication include ones for short distance and ones for long distance. In the optical LAN, a module which is compatible to both applications is required. In general, the fiber for short distance supplies a strong light signal and a fiber for long distance supplies a weak light signal. Accordingly, an AGC function is required to cope with the signals from those fibers. Two prior art techniques to attain the AGC are known. In one approach, an external control circuit is provided to control a gain of a pre-amplifier. (For example, Japanese Patent Application No. 1-217967). In the other approach, this function is provided in the amplifier and the internal circuit produces an average level and varies a bias voltage of the amplifier in accordance with the average level.
However, in the approach to control the gain of the pre-amplifier, heat generation is not negligible, power consumption is high and noise is high. In the approach to change the bias voltage of the amplifier by the internal circuit of the amplifier, additional problems of complexity of circuit configuration and low yield of the manufacture of the OEIC arise.
Further prior art optical switches (a) use a two-dimensional planar wave guide formed on a LiNbO.sub.3 surface or (b) convert a light signal to an electrical signal and process it by an associated electrical circuit.
One photo-logic device (c) directs parallel light signals of vector components to two orthogonal planes of a nonlinear optical material and takes out a secondary harmonic generated at a cross-point of the parallel light signals as a matrix of an optical AND signal. Further, an optical neuro chip used in the photo-operation (d) uses a mask having a transmissibility which is proportional to a synapus load sandwiched between a belt-shaped light emitting diode array and belt-shaped photo-sensing diode array (Optics Letters, Vol. 14, No. 16, Aug. 15, 1989), and an optical memory used for the optical operation (e) uses a parallelly connected pn structure (Optics Letters, Vol. 15, No. 13, Jul. 1, 1990).
However, in the optical switch of (a), the absorption in the LiNbO.sub.3 wave guide is not negligible and a transmissive characteristic is not good. In the optical switch of (b), the structure is complex because the photo-sensing device, IC and light emitting device are integrated.
In the photo-logic device of (c), the output optical AND signal is weak because the nonlinear optical material is used and only two-dimensional processing is permitted. In the devices of (d) and (e), they are not photo-logic devices by themselves and their burden to the associated electrical circuit is large.